Fuel use in new cars is 23 per cent above car manufacturers' claims – as are emissions levels – according to An Taisce, the National Trust for Ireland, because of gaping loopholes in the EU's procedures for testing new cars.

"The reason motorists don't see fuel economy claims translate into reality is that carmakers are gaming the vehicle testing system," according to James Nix, of An Taisce.

He adds that "it's an easy system to manipulate because manufacturers do things during the fuel cycle test that drivers would never do".

Examples of what manufactures do include: taping over the gaps between the doors; over-inflating the tyres; testing at altitude; disconnecting the alternator; using Formula 1-grade lubricant, and removing items that can be detached from the car, right down to the mirror on the passenger side.

All these practices were revealed in a report published by Transport & Environment, an EU-wide federation working to reduce energy use and emissions in transport, of which An Taisce is a member.

"Car manufacturers can massage fuel economy results without being in formal breach of EU rules," says An Taisce.

The environmental organisations have called on the EU to take four steps:

• Close the loopholes immediately and retest cars under the corrected system.

• Have globally consistent fuel and emissions testing by 2016.

• Make sure that carmakers do not pay test centres directly but instead contribute to a central fund.

• Reform car labelling in the EU's Directive so that it reflects real-world results.